How can I locate these marriage records?

I am searching for marriage records for a couple married in 1906 in Glassport, Allegheny, Pennsylvania. They are my great grandparents and I require this information in order to apply for my grandmother’s Irish citizenship by ancestry.

If anyone could point me in the correct direction for getting a hold of this certificate, I would appreciate it. From research I haven’t been able to find a phone number or obvious place to order it from. Thank you!